Security element

ABSTRACT

A security element ( 2 ) in the form of a laminate can be used for the authentication of a document ( 1 ). The laminate has at least a transparent protective layer, a transparent lacquer layer and an adhesive layer, the lacquer layer being arranged between the protective layer and the adhesive layer. An interface in the form of a reflection layer separates the adhesive layer and the lacquer layer. The interface is divided into regions of a pattern ( 25 ) with flat surface portions and with relief structures which are formed in the lacquer layer. The flat surface portions form background surfaces ( 3 ) providing flat mirror surfaces for light which is incident into the laminate, while the regions with the relief structures of a predetermined, optically effective structure depth form pattern elements ( 4 ). The relief structures of the pattern elements ( 4 ) absorb the incident light. In the reflected light of the reflecting background surfaces ( 3 ) the dark pattern elements ( 4 ) form a strong contrast and the pattern ( 25 ) is clearly visible. In directions other than that of the reflected light the contrast between the background surfaces ( 3 ) and the pattern elements ( 4 ) disappears so that a copier apparatus reproduces the background surfaces ( 3 ) and the pattern elements ( 4 ) as black surfaces.

[0001] The invention relates to an optically diffractive securityelement, as set forth in the classifying portion of claim 1.

[0002] Such security elements are used for the authentication ofdocuments such as value-bearing papers or bonds, cheques, banknotes,credit cards, passes and identity cards of all kinds, entrance tickets,driving licences and so forth, the security element being for example inthe form of a thin layer composite or laminate, which is fixed on thedocument by adhesive.

[0003] Modern copiers for coloured copies represent a serious potentialdanger for documents which are produced by a printing procedure, becausethe visual differences between the original and the copy are so slightthat only an expert equipped with the appropriate aids can distinguishthe original from the copy, in which respect it is often necessary torefer to other criteria such as intaglio printing, a watermark,fluorescence, optically variable security elements with diffractionstructures and so forth, apart from the appearance of the printed image.

[0004] It is known from EP 0 522 217 B1 that reflective foil portionsarranged on a document implement good protection against unauthorisedcopying of such documents. The difference between the original with thereflective foil portions and a copy can be clearly seen as the copyingmachines reproduce reflective surfaces as black. It will be appreciatedhowever that reflective foils are readily available on the market. Theblack surfaces in such copies can therefore easily have reflective foilstuck over them, in order to make the copy appear more genuine.

[0005] DE 44 10 431 A1 describes further developments of theabove-described foil portions. The security element is a foil portionwhich is cut from a laminate, with a flat, mirroring reflection layer.The reflection layer is removed in surface portions which form anindividual identification on the surface of the foil portion, so that ablack layer arranged under the reflection layer becomes visible. On thecopy produced by the copier machine, the black identification disappearsin the reproduction of the remaining mirror surface, as the surfaceportions in which the reflection layer is removed and the mirror surfacewhich has remained behind in the copy appear uniformly black. Anothersecurity element, instead of the flat mirror surfaces, has a hologramstructure with the identification and, in the copying procedure, behaveslike the diffraction structures which are discussed in the nextparagraph. In the copy therefore, the identification can be detected inthe copied image of the hologram.

[0006] It is also known for example from GB 2 129 739 B for valuabledocuments to be provided with an optically variable security elementhaving diffraction structures (for example holograms, mosaic-likesurface patterns comprising diffractive surface elements, for example inaccordance with EP 0 105 099 A1, EP 0 330 738 A1, EP 0 375 833 A1, andso forth). Those security elements have a pattern or image which changesin dependence on the viewing condition. From the point of view ofunauthorised persons, those security elements can be imitated only athigh cost. Unfortunately, the colour copy of the document reproduces oneof the patterns or images of the security element, which is visible inthe original under the viewing condition which is fixed in the copierfor the imaging procedure. It will be appreciated that it is no longerpossible to see any change in the pattern or image in the copy, upon avariation in the viewing condition, but if the receiver is not payingattention, a copy can easily be considered to be the genuine document.

[0007] Embodiments of the laminate for the security elements andmaterials which can be used for that purpose are described in EP 0 40146 A1 and U.S. Pat. No. 4,856,857.

[0008] The object of the present invention is to provide an inexpensive,optically variable security element which cannot be reproduced by acopier apparatus and which also cannot be copied with holographicmethods.

[0009] In accordance with the invention that object is attained by thefeatures recited in the characterising portion of claim 1. Advantageousconfigurations of the invention are set forth in the appendant claims.

[0010] Embodiments of the invention are described in greater detailhereinafter and illustrated in the drawing in which:

[0011]FIG. 1 shows a document,

[0012]FIG. 2 shows the document when tilted about an axis,

[0013]FIG. 3 shows a view in cross-section of a security element,

[0014]FIG. 4 shows the interface of a relief structure,

[0015]FIG. 5 shows a first observation condition,

[0016]FIG. 6 shows a second observation condition,

[0017]FIGS. 7a and 7 b show the security element with grey stages, and

[0018]FIG. 8 shows a relief structure.

[0019] Referring to FIG. 1, reference 1 denotes a document, 2 denotes asecurity element, 3 a background surface, 4 a pattern element and 5 anotional tilt axis which is in the plane of the document 1. The document1 is illuminated using directed artificial light laterally andinclinedly from above and is viewed perpendicularly from above. Thesecurity element 2 is fixed on the document 1. For identificationpurposes, the security element 2 has a pattern 25 comprising the patternelements 4 which are surrounded by the background surfaces 3. In orderto make the drawing in FIG. 1 clearer, the pattern 25 comprises a singlepattern element 4 and forms a simple “V” sign. A practical embodimentinvolves the arrangement of a plurality of the background surfaces 3 andthe pattern elements 4, with respect to the pattern 25. Under thespecified illumination and observation conditions, the pattern 25 is notvisible to an observer as there is no contrast between the patternelement 4 and the background surface 3, and both surfaces, both thebackground surface 3 and also the pattern element 4, appear dark, forexample metallically matt. In diffuse daylight or in diffuse roomlighting in contrast and under certain illumination conditions which areset forth hereinafter, the pattern element 4 stands out as being dark,from the light background surface 3, and is therefore clearly visible tothe observer.

[0020] If, as shown in FIG. 2, the document 1 with the security element2 is tilted about the tilt axis 5 in such a way that the backgroundsurface 3 reflects light into the eye of the observer, then the observercan recognise the pattern 25 as the pattern element 4 remains dark andstands out from the background surface 3, with a high level of contrast.Under that observation condition, the reflection condition for theobserver is fulfilled. Rotation of the security element 2 in its planedoes not change the appearance of the pattern 25 in the reflectioncondition, from the point of view of the observer, that is to sayazimuthal orientation of the security element 2 is not to beimplemented.

[0021]FIG. 3 shows the security element 2 (FIG. 2) in cross-section,wherein the section plane contains for example the tilt axis 5 (FIG. 2).The security element 2 comprises a layer composite or laminate 6comprising a plurality of layers 7, 8, 9 and 11. Examples in regard tothe structure of the laminate 6 and the materials used for same are tobe found in EP 0 401 466 A1 and U.S. Pat. No. 4,856,857.

[0022] In the simplest case the laminate 6 includes at least aprotective layer 7, an adhesive layer 8 and a lacquer layer 9 arrangedbetween the protective layer 7 and the adhesive layer 8. The adhesivelayer 8 joins the security element 2 to the document 1. An interfacebetween the adhesive layer 8 and the lacquer layer 9 reflects light 10which is incident through the cover layer 7 and the lacquer layer 9 ifthe refractive index at the boundary layer suddenly changes at thetransition from the lacquer layer 9 to the adhesive layer 8. With thematerials in Table 6 of U.S. Pat. No. 4 856 857 the difference in therefractive indices is too small to achieve a strong reflection. Thereflection capability is therefore increased by a reflection layer 11which is arranged at the interface and which is a thin layer (<0.4micrometer) comprising a metal or a metal coated with a suitableinorganic dielectric layer, the dielectric layer being arranged on theside of the metal, that is towards the incident light 10.

[0023] The materials for the reflection layer 11 are contained Table 1to 5 of U.S. Pat. No. 4,856,857; Tables 1 to 6 are expresslyincorporated into this description. Tellurium which is not mentioned inTable 5 is also suitable for the reflection layer 11. The incident light10 means daylight or visually visible polychromatic light of wavelengthsof between 380 nm and 780 nm.

[0024] In another embodiment of the laminate 6 the surface of the coverlayer 7 of the laminate 6; that is remote from the lacquer layer 9, isconnected to a carrier band or strip 13 by means of a separating layer12 in order to facilitate transfer of the fragile laminate 6 onto thedocument 1. The carrier band 13 of paper or a plastic foil, for examplePC or PETP can be removed after the laminate 6 has been glued in placeso that the pattern 25 (FIG. 2) is visible through the protective layer7 and the lacquer layer 9. In that respect attention is directed to GB 2129 739 B which has already been referred to above.

[0025] As can be seen from FIG. 3 a relief structure 14 with ageometrical profile depth p is formed into the lacquer layer 9, in theregion of the pattern elements 4. In the region of the backgroundsurfaces 3 the lacquer layer 9 is formed smooth and flat and is parallelto the other layers of the laminate 6. The material of the adhesivelayer 8 fills the depressions of the relief structure 14. The interfacewith or without the additional reflection layer 11 follows both therelief structure 14 and also mirror planes of the background surfaces 3.

[0026] The relief structure 14 is a cross grating comprising two basegratings of periods d smaller than a limit wavelength λ at theshort-wave end in the spectrum of visible light, that is to say betweenλ=380 nm and λ=420 nm and has an optically effective structure depth h,that is the profile depth p multiplied by the refractive index of thelacquer layer 9, preferably in the range of between h=50 nm and h=500nm. Such relief structures 14 absorb almost all visible light 10 whichis incident on the pattern elements 4 and scatter a small fraction ofthe incident light 10 back into the half-space above the pattern element4. The percentage of the absorbed light 10 is non-linearly dependent onthe structure depth h and can be controlled by means of the choice ofthe structure depth h in the above-mentioned range of between 50% andabout 99%, in which respect the shallower the relief structure 14 thecorrespondingly more incident light 10 is scattered back and thecorrespondingly less light 10 is absorbed. The specified percentagesapply for the relief structure 14 with a reflection layer 11 of forexample aluminium. Adjoining regions of the pattern elements 4 withvarious structure depths h therefore exhibit a grey graduation.

[0027] The embodiment of the relief structure 14 which is shown in FIG.4 is a cross grating formed by two sinusoidal base gratings which crossat a right angle. The sine function of the first base grating, whichextends along the co-ordinate x, is of a period d_(x) and an amplitudeh_(x) while the sine function of the second base grating which extendsalong the co-ordinate y is of a period d_(y) and an amplitude h_(y).Over the plane defined by the co-ordinates x and y the interface h(x, y)formed by the cross grating in the laminate 6 (FIG. 3) follows forexample the function:

h(x, y)=[h _(x) +h _(y)]·sin²(πx/d _(x))·sin²(πy/d _(y)).

[0028] Other embodiments involve h(x,y)=h_(x)·sin²(πx/d_(x))+h_(y)·sin²(πy/d_(y)), with rectangular orpyramid structures being used as the interface h(x, y).

[0029] In an embodiment the two periods d_(x), d_(y) and the structuredepths h_(x), h_(y) are the same, while in other embodiments they aredifferent. The structure depth h=[h_(x)+h_(y)] can be selected to begreater than the period d, but the relief structure 14 is difficult toproduce, with the present-day manufacturing methods. The interface h(x,y) is like an egg carton and is shown in FIG. 4.

[0030] Referring to FIG. 5, the optical behaviour of the securityelement 2 with a first observation direction will now be discussed. Theincident light 10 forms an angle of about 40° with the normal 15 to theplane of the security element 2. In an example the pattern elements 4with the above-described relief structure 14 absorb in the visible rangeup to 95% of the incident light 10, the remainder is scattered. Thereflective background surface 3 in contrast absorbs only about 10% ofthe incident light 10 and reflects the rest. As surface portions of thepattern elements 4 adjoin the reflective background surfaces 3, theobserver therefore has such a strong contrast that the pattern elements4 arranged on a predetermined background surface 3 of the securityelement 2 in the predetermined pattern 25 can be easily recognised asinformation. The pattern 25 represents a logo, a text, an image oranother graphic character.

[0031] The drawing in FIG. 5 corresponds to the illumination conditionsin the copier apparatus. Depending on the respective model of the copierapparatus, the directed light 10 of the copier apparatus which isincident on the document 1 and the security element 2, forms the angleof incidence α in the range of about 40° to 50° to the normal 15. Thedocument 1 scatters the incident light 10 into the entire half-space. Asa result scattered light passes into a light receiver 16 of the copierapparatus, which is arranged in the direction of the normal 15. Incontrast thereto the light 17 which is reflected from the backgroundsurface 3 is deflected at the same angle α in accordance with the law ofreflection into a viewing direction 18 of the observer 19 and does notpass into the light receiver 16. If the light 10 is incident on thepattern element 4 at the same angle of incidence α, the incident light10 in contrast is practically absorbed; both the light receiver 16 andalso the observer 19 register no light from the pattern element 4. Thepattern element 4 is therefore dark.

[0032] The background surfaces 3 form the flat mirror surfaces of thepattern 25, for the light 10 which is incident in the laminate 6, whilethe pattern elements 4 as absorber surfaces swallow up the major part ofthe incident light 10. Therefore, in the reflected light 17, theobserver 19 recognises the background surfaces 3 in the form ofintensively light surface portions and the pattern elements 4 as darksurface portions of the pattern 25. In directions other than that of thereflected light 17, the security element 2 scatters only a small part ofthe incident light 10. The levels of intensity per unit of surface areaof the light which is scattered at the background surfaces 3 and thepattern elements 4 are practically of the same magnitude so that thereis no contrast between the background surfaces 3 and the patternelements 4. In the case of illumination with the directedly incidentlight 10, the pattern 25 formed from the background surfaces 3 and thepattern elements 4 is recognisable only in the light 17 reflected withspecular reflection, in contrast to a black-and-white image which isproduced by a printing procedure.

[0033] In the copier apparatus the background surface 3 and the patternelement 4 projects such a small projection of the incident light 10 intothe light receiver 16 that the copier apparatus indiscriminatelyreproduces the background field 3 and the pattern element 4 as blacksurfaces. The advantage of this security element 2 is that the copierapparatus cannot reproduce the information represented by the patternelement 4 while the observer 9 who, when using directedly incident light10, almost automatically tilts the security element 2 in such a way thathe views the background surface 3 in a reflection mode, can see theinformation of the pattern element 4 with a high level of contrastagainst the background surface 3. In that way the security element 2 canbe easily distinguished by an attentive observer from reflecting metalfoils on good coloured copies of the document 1. A further advantage isformed by the use of the relief structure 14 in the security element 2with the periods d_(x) (FIG. 4) and d_(y) (FIG. 4) which are shorterthan the wavelengths of the coherent light sources which can be used forholographic copying methods; it is therefore not possible to produce acopy of the security element 2 with the holographic methods.

[0034]FIG. 6 shows a second illumination condition for the two observers19, 20 of the security element 2. A polychromatic radiation source 21,for example a halogen lamp, an incandescent lamp and so forth, isarranged above the second observer 20 and emits the incident light 10onto the pattern element 4 at a large angle of incidence α of about 60αto 80°. The first observer 19 sees the pattern 25 (FIG. 2) of thepattern elements 4 in front of the background 3 (FIG. 5) at thereflection angle α, as referred to above. If the periods d_(x) (FIG. 4),d_(y) (FIG. 4) of the relief structure 14 are in the region of a halfand whole limit wavelength λ; that is to say λ≧d≧λ/2, wherein d=d_(x) ord_(y) respectively, a part of the incident light 10 is deflected at alarge diffraction angle β into the minus first order, as diffractedlight 22. The second observer 20 can recognise the diffracted light 22.The diffracted light 22 includes the short-wave portion of the visuallyvisible spectrum of the electromagnetic radiation. The diffracted light22 is therefore dependent on the diffraction angle β and the periodsd_(x), d_(y) in a blue-green to violet colour. The colour of thediffracted light 22, which is observed at a predetermined diffractionangle β relative to the normal 15, is also dependent on the azimuth, inrespect of its intensity. Note: in the foregoing consideration therefractive influence of the protective layer 7 has been disregarded.

[0035] In contrast, the first observer 19 is looking in the direction ofthe reflected light 17 and sees the background surfaces 3 as shinilybright surface portions and the pattern elements 4 as dark surfaceportions of the pattern 25.

[0036] If the period d_(x) or d_(y) is less than λ/2, the diffractedlight 22 can no longer be seen by the second observer 20, in thedirection of the co-ordinate x or y respectively, as the reliefstructure 14 no longer diffracts visible light 22. The first observer 19who is observing the security element 2 at the reflection angle α seesthe pattern elements 4 unchanged in a dark-brown to black colour, underthese conditions.

[0037] The colour of the pattern elements 3 which are visible at thereflection angle α depends on the nature of the reflection layer 11 asvarious combinations of the materials in and at the reflection layer 11do not uniformly reflect the incident light 10 in the entire spectralrange of the visible electromagnetic radiation. Deep-black patternelements 3 advantageously have a gradual transition in respect of therefractive index from the lacquer layer 9 to the reflection layer 11;the transition is produced by means of at least one layer of aninorganic dielectric 23 between the lacquer layer 9 and a metal layer 24of the reflection layer 11. For the flat mirror surface of thebackground surfaces 3, the reflection layer 11 formed from thedielectric 23 and the metal layer 24 does not have a noticeable effect.In the case of the relief structure 14 in contrast, that reflectionlayer 11, as a consequence of interference phenomena, causes almostcomplete extinction of the incident light 10, which occurs in particularuniformly over the entire spectral range of the visible electromagneticradiation. An example has a 50 nm thick layer of the dielectric 23 ofZnS and 100 nm of aluminium as the metal layer 24. A further advantageis the structure depth h which is increased by the high refractive indexfor ZnS of n=2.4, in relation to the refractive index of the lacquerlayer 9 of n=1.5, with the profile depth p of the relief structure 14remaining the same.

[0038] Besides the grey graduations with pattern elements 4 withdifferent structure depths h the grey graduation in an embodiment of thesecurity element 2 is produced by means of rastering of varying density,with raster dots of less than 0.4 nm in dimension. In that respect it isimmaterial whether the raster dots are arranged as the background field3 in a pattern element 4 or as the pattern element 4 in the backgroundfield 3.

[0039]FIGS. 7a and 7 b show further examples for the production of greystages or steps within a security element 2 from the dark patternelement 4 to the brightly shining background field 3. FIG. 7a involvesusing raster dots of various sizes in a fixed raster of a maximum 0.5 mmspacing, corresponding to the grey stage. In a slightly lightened zone26 the raster dots touch, in a lightened zone 27 the raster dots are ofa mean dimension of about 0.25 mm while in a slightly darkened zone 28the raster dots are about 0.15 mm. In FIG. 7b, instead of the dotraster, there is a line raster with a maximum 0.5 mm spacing. Acorresponding line width here affords the grey graduation in the zones26 (FIG. 7a) to 28 (FIG. 7a).

[0040] In one of the zones 26 to 28 the raster dots of the patternsurfaces 4 are of the same dimensions. A very fine grey graduation isachieved by means of the suitably stepped structure depths h in therelief structures 14 (FIG. 6), which is adequate for the reproduction ofa black-and-white photograph.

[0041]FIG. 8 shows two patterns 25 of the security element 2 as a simpleexample. In the upper half of the security element 2 the pattern 25comprises a band 29 with a star 30. The band 29 is formed from the darkpattern element 4. The area around the band 29 and the star 30 form thelight background surfaces 3. Without limitation in respect of theforegoing description, the background surfaces 3 and the patternelements 4 are interchangeable, as is shown in the lower half of thesecurity element 2.

[0042] The security element 2 in FIG. 1 will be still more difficult tocounterfeit if the pattern 25 forms a background for a mosaic surfacepattern 31 with diffraction structures who spatial frequencies are ofvalues in the range of 300 lines per mm to 2000 lines per mm. Suchmosaic surface patterns 31 are known from above-mentioned EP 0 105 099A1, EP 0 330 738 A1 and EP 0 375 833 A1. The content of those patentspecifications is hereby incorporated into this description.

1. A security element with a pattern comprising surface portions and inthe form of a laminate for authentication of a document, which includesat least a transparent protective layer, a transparent lacquer layer andan adhesive layer, wherein the lacquer layer is arranged between theprotective layer and the adhesive layer and the refractive indexabruptly changes at the interface between the adhesive layer and thelacquer layer and the surface portions of the pattern are composed ofbackground surfaces and pattern elements, wherein in the region of thebackground surfaces the lacquer layer is smooth and flat in form and inthe region of the pattern elements relief structures with apredetermined, optically effective structure depth are formed into thelacquer layer, the background surfaces are flat mirror surfaces forlight incident into the laminate, and the relief structures are crossgratings formed from base gratings with periods and the periods areshorter than a predetermined limit wavelength at the short-wave end inthe spectrum of visible light so that the pattern elements absorb andscatter the incident light, wherein in each relief structure the ratioof the absorbed and the scattered light is predeterminedly dependent onthe optically effective structure depth prevailing in the reliefstructure.
 2. A security element according to claim 1, wherein the crossgrating of the relief structure is composed of two base gratings withthe periods which are arranged substantially at a right angle relativeto each other.
 3. A security element according to claim 1, wherein thebase gratings are sinusoidal.
 4. A security element according to claim1, wherein at least one of the periods is longer than half the limitwavelength but shorter than the limit wavelength.
 5. A security elementaccording to claim 1, wherein the limit wavelength is selected to be inthe range of between 380 nm and 420 nm.
 6. A security element accordingto claim 1, wherein the periods of the two base gratings are of the samevalue.
 7. A security element according to claim 1, wherein the valuesfor the optically effective structure depth of the relief structures areselected to be in the range of 50 nm to 500 nm.
 8. A security elementaccording to claim 1, wherein the reflection layer contains a metal fromthe group consisting of aluminum, silver, gold, chromium, copper, nickeland tellurium.
 9. A security element according to claim 8, wherein thereflection layer, on the side of the metal layer that is towards thelacquer layer, has at least one layer of an inorganic dielectric.
 10. Asecurity element according to claim 9, wherein the layer of theinorganic dielectric comprises ZnS and the metal layer comprisesaluminum.
 11. A security element according to claim 1, wherein thepattern has zones with grey stages and that the pattern elements of thezones with various grey stages differ by the optically effectivestructure depth of the relief structures.
 12. A security elementaccording to claim 1, wherein the pattern has zones with grey stages,that the pattern elements have identical values with respect of to theoptically effective structure depth and that the zones differ byrastering of varying density of raster dots of dimensions of less than0.4 mm.
 13. A security element according to claim 1, wherein the patternforms a background for a mosaic surface pattern comprising diffractionstructures with spatial frequencies in the range of 300 lines per mm to2000 lines per mm.